1. Technical Field
This invention relates to electroacoustic transducers, such as microphones, and particularly to capacitive electroacoustic transducers fabricated in batches by means of a wafer manufacturing process.
2. Background Art
Capacitive electroacoustic transducers are widely used for the measurement of static and dynamic pressures. Traditionally, these capacitive transducers, such as employed in a microphone, have been made in such a manner that one electrode of a capacitor structure is formed by an electrically conductive diaphragm. This diaphragm is disposed adjacent to, but insulated from, a stationary electrode forming the other electrode of the capacitor structure. The two electrodes are spaced apart with an air gap in-between. A relatively high DC bias voltage is then applied between the electrodes. Variations in the electrode spacing caused by deflections of the diaphragm in response to the force of acoustic wave energy incident on the diaphragm, produce a change in capacitance. A detection network is connected to the capacitive transducer such that the change in capacitance is detected and transformed into an electrical signal proportional to the force of the acoustic wave energy applied to the diaphragm.
The sensitivity and performance of a capacitive electroacoustic transducer is closely tied to the at-rest spacing between the diaphragm and the stationary electrode. Thus, this spacing must be accurately controlled. To achieve accurate spacing, close machining tolerances are required for the parts making up the transducer. The required tolerances can be extremely difficult to hold in production. As a result, these devices are often hand crafted from machined parts in an attempt to meet the response and sensitivity characteristics imposed by the particular application in which the transducer is to be employed. This hand crafting tends to increase the cost of the transducers. Additionally, each transducer so produced exhibits a slightly different response in phase and magnitude.
The sensitivity and response of a capacitive electroacoustic transducer is also closely tied to its thermal stability. This thermal stability is partially dependent upon the change in the separation between the diaphragm and the stationary electrode caused by expansion or contraction of the transducer components when subjected to changing temperatures. The critical electrode spacing in existing capacitive transducers has been difficult to maintain over a widely varying temperature environment. This is especially true where the differential axial expansion length of the components is large in the first place. For instance, many existing transducers have expansion lengths on the order of 0.25 inch. Large expansion lengths mean that expansion and contraction of the transducer elements produce significant changes in the electrode separation distance. A significant change in this separation distance alters the response of the transducer. Additionally, changes in the tension on the diaphragm resulting from differing rates of expansion for the case than for the diaphragm, also affect the thermal stability of the transducer. When the tension of the diaphragm is allowed to change with temperature, the sensitivity of the transducer is altered.
Therefore, what is needed is a capacitive electroacoustic transducer which can be batch produced with consistent and reproducible response and sensitivity performance characteristics, and which maintains these characteristics even over a widely varying temperature environment.